The present invention relates generally to fiber optic communications systems, and more particularly, to the use of higher order modulation techniques to transmit information on passbands of a dispersion-limited fiber link.
The microwave response of an optical link is degraded by dispersive fiber. Laying fiber in the ground is the predominant cost driver in the fiber communication business. Dispersion causes passbands in the link. These passbands vary with distance. As the link length increases, the passbands narrow and have a smaller period.
The response of the dispersive fiber link can be flattened by several known techniques. One technique involves introducing dispersion of an equal magnitude but opposite sign at a receiver at the opposite end of the fiber. However, these techniques are expensive and introduce additional optical loss.
Non-dispersive fiber has been recently developed. However, almost all of the fiber in the ground today is dispersive. Consequently, techniques that can enhance the information carrying capacity of existing fiber are extremely desirable.
Various patents have been issued relating to transmission of information over a fiber optic link that were uncovered in a search relating to the present invention. These patents disclose apparatus and methods relating to canceling distortion, multiplexing and demultiplexing signals, communicating using multiple polarizations, correction of nonlinear effects, optical transmission using optical phase conjugation, transmitting signals having different wavelengths over a same optical fiber, dispersion compensation, fault signaling, amplifiers, and filters. However, none of the patents uncovered in the search disclosed using higher order modulation techniques to transmit information on passbands of a dispersion-limited fiber link to increase the information carrying capacity of the link.
Therefore, it would be advantageous to have systems and methods that increase the information carrying capacity of fiber optic communication links.
The present invention provides for systems and methods that use higher order modulation techniques to transmit information on passbands of a dispersion-limited fiber optic link. The present invention provides for the transmission of data on previously unused passbands of the dispersion-limited optic link, thus increasing the total capacity of the fiber optic link.
The microwave response of a typical optical link is degraded by dispersive fiber, and there are nulls and passbands in the response. The prior art relating to optical link design utilizes only on-off keying in the first passband. The present invention, however, provides for the transmission of higher order modulated carrier frequencies centered within the higher passbands. Such higher order modulated carrier frequencies include M-ary amplitude shift keyed (ASK) modulation, M-ary phase shift keyed (PSK) modulation, and M-ary quadrature amplitude modulation (QAM), and the like. Use of these carriers does not preclude simultaneous use of the first band as in prior art. Thus, the present invention combines a multiplicity of encoded carriers into a composite microwave spectrum that is tailored to the passbands of the optical link.
Each of the carriers transmit additional data over the dispersive fiber compared to what is normally transmitted using the first band alone. This increases the information (data) carrying capacity of the fiber communication link employing the dispersive optical fiber.
An exemplary system comprises a plurality of modulators that modulate a plurality of data streams. The first modulator is an amplitude shift keyed (ASK) modulator, using either on-off keying or multiple amplitude steps. The remaining modulators encode their data streams on a carrier 12 using bandwidth efficient modulation. Numerous M-ary amplitude and phase shift keying methods may be used to produce the bandwidth efficient modulation.
A plurality of frequency converters are used to convert the carriers to a plurality of frequencies that are respectively centered within a passband of a fiber optic link. A combiner is used to combine the signals into a composite microwave spectrum that is transmitted over the fiber optic link. At the receiving end of the link, a splitter and a plurality of bandpass filters are used to separate the recovered spectrum is into its individual data streams. A plurality of frequency converters and a plurality of demodulators are used to extract the originally transmitted data from each data stream.
An exemplary method comprises the following steps. Data for transmission is modulated using a microwave carrier signal to produce multiple carrier signals having a first carrier frequency. The carriers are converted to appropriate frequencies, one for each passband of the fiber optic communication link. The frequency converted modulated carriers are then combined to provide a composite microwave signal. The composite signal is transmitted over the fiber optic communication link. The composite signal is reproduced at a receiving end of the fiber optic communication link. The individual signals are separated out from the composite signal, and are converted back to the original carrier frequency. The signals are then demodulated to generate the originally transmitted data.